1. Field of Invention
The present invention relates to multilayer glass structures, and particularly a method of making multilayer glass structures decorated using crystallizing enamels.
2. Description of Related Art
Crystallizing glass enamel compositions are well known in the art. They can be used for a variety of applications such as, for example, decorative coatings for glassware, chinaware, and the like. They are especially useful in forming colored borders around glass sheets used for automotive windshields, sidelights and backlights. The colored borders enhance appearance as well as prevent UV degradation of underlying adhesives. The crystallizing glass enamels are reactive compositions for they contain components that react and crystallize upon firing.
In general, these enamel compositions consist mainly of a glass frit, a colorant and an organic vehicle. They are applied to a substrate, for example, a sheet of glass, and subsequently fired to burn off the organic vehicle and fuse the frit thus bonding the enamel coating to the substrate. Glass sheets for automotive use are generally coated with the enamel composition and then subjected to a forming process at elevated temperatures. During this treatment the enamel melts and fuses to the glass substrate and the glass is formed into a desired final shape. Such compositions may also be applied to one layer of a multilayer laminate (such as a safety glass windshield) prior to stacking the layers together, whereby the pigment/color is in the interior of the resulting multilayer laminate.
In such case, after application of the enamel by screen printing, for example, the wet film is dried or cured at low temperatures or with UV light to remove the solvents and produce a dried green enamel where the particles are held together by higher molecular weight organic binder molecules. After producing the green enamel layer, it can be overprinted with a conductive silver layer, and at least three additional heating steps at higher temperature are required. The first heating burns off the higher temperature higher molecular weight organic binder molecules and adheres the enamel composition to the first substrate layer. A second heating allows a second glass substrate to be mated and bent together with the first. After insertion of a vinyl sheet (e.g., polyvinylbutyral) between the first and second glass sheets, a third lower temperature heating is then needed to fuse the two glass sheets and the vinyl sheet to form a glass monolith, i.e., a laminated glass windshield pane.
When the glass substrates are stacked using a conventional crystallizing enamel composition when still green, upon firing to form a mated set, the result can be incomplete removal (i.e., combustion) of the higher temperature organic binders, which can cause significant carbon entrapment and degradation of the desired black color of the enamel. In addition, the enamel on the bottom sheet can undesirably adhere to the top sheet, causing damage to either or both.
U.S. Pat. No. 5,443,669 to Tünker discloses a method of making decorated multilayer glass structures wherein both panes of glass are fired together. However, the method of Tünker can lead to the sticking of the panes for such method does not employ a crystallizing glass enamel. Additionally, the method of Tünker employs an inorganic binder comprising special silica gels that do not easily mix with the other components of the enamel composition. Moreover, the Tünker patent teaches the use of lead containing oxidizers or oxidizers containing low levels of oxygen that can insufficiently react with conventional binder materials.